Directional control has become increasingly important in the drilling of subterranean oil and gas wells, for example, to more fully exploit hydrocarbon reservoirs. Two-dimensional and three-dimensional rotary steerable tools are used in many drilling applications to control the direction of drilling. Such steering tools commonly include a plurality of force application members (also referred to herein as blades) that may be independently extended out from and retracted into a housing. The blades are disposed to extend outward from the housing into contact with the borehole wall and to thereby displace the housing from the centerline of a borehole during drilling. The housing is typically deployed about a shaft, which is coupled to the drill string and disposed to transfer weight and torque from the surface (or from a mud motor) through the steering tool to the drill bit assembly.
While such steering tools are conventional in the art and are known to be serviceable for many directional drilling applications, there is yet room for further improvement. In particular, directional drilling operations may be enhanced by improved control of the steering tool. The ability to quickly and reliably transmit steering tool commands from an operator at the surface to a downhole steering tool may advantageously enhance the precision of a directional drilling operation. For example, the ability to continuously adjust the drilling direction by sending commands to a steering tool may enable an operator to fine tune the well path based on substantially real-time survey and/or logging-while-drilling data.
Prior art communication techniques that rely on the rotation rate of the drill string to encode steering tool commands are known. For example, Webster, in U.S. Pat. No. 5,603,386, discloses a method in which the absolute rotation rate of the drill string is utilized to encode tool commands. Webster discloses a pressure sensor, located on the output line of a hydraulic pump, or alternatively a Hall-effect sensor, to assess the rotational speed of the drill string. Barron et al., in U.S. Publication No. 2005/0001737, disclose an encoding scheme in which a difference between first and second rotation rates is utilized to encode commands. A magnetic marker located on the driveshaft and a Hall-effect sensor deployed on the housing are utilized to determine rotation rate of the drill string. While these prior art approaches are known to be serviceable, they may be improved upon for certain directional drilling application.
For example, in some applications, steering tool commands may be advantageously transmitted downhole immediately after a new section of drill pipe has been added to the drill string and an MWD survey has been received at the surface. In such applications, the housing is known to sometimes rotate with respect to the borehole (since the drill bit is typically off bottom and the blades may be somewhat disengaged from the borehole wall). Rotation of the housing, if not accounted, can introduce errors into the aforementioned drill string rotation rate measurements (which measure the rotation rate of the shaft with respect to the housing), thereby potentially resulting, for example, in miscommunication of a steering tool command. Such miscommunication requires retransmission of the command, which wastes valuable rig time. Miscommunication of a steering command may also occasionally have more serious consequences, such as drilling the well in the wrong direction.
Furthermore, drilling conditions are often encountered in which the drill string sticks and/or slips in the borehole. This is a condition known in the art and commonly referred to as stick/slip. In stick/slip situations, precise measurement of the drill string rotation rate is often problematic because the rotation rate is not constant in time. Stick/slip conditions therefore present difficulties to the timely and accurate transmission of steering tool commands downhole.
Other downhole tools, including, for example, MWD and LWD tools, may also benefit from the measurement of instantaneous (substantially real-time) rotation rates. For example, such measurements may improve the reliability of survey and LWD data.
Therefore, there exists a need for an improved mechanism for measuring substantially real-time rotation rates of downhole tools. For example, for steering tool embodiments, a mechanism that enables substantially instantaneous rotation rates to be measured would advantageously enhance communication between the surface and the downhole steering tool.